No significant advances have been made presenting new concepts in sheet-fed printing systems for decades. Printing systems designed for the sheet-fed printer are basically the same and allow printing on one side of the sheet at a time, requiring sheets to be turned over and rerouted through the press for single or multi-color perfecting. Sheets are progressively and meticulously transferred in serpentine fashion about transfer and impression cylinders and hopefully registered from one cylinder to another and from one printing unit to another until finally they emerge as a printed product. Printing units must be synchronized for color register through numerous drive and idler gears and consequently presses are extremely complex, massive units which are very expensive to manufacture because of numerous transfer and printing cylinders and mechanisms related thereto.
One or two color sheet-fed perfectors have been developed heretofore. However, these machines were specifically designed for specific jobs, such as mass production of paperback books, and are totally unsuitable for high speed production of four-color process printing on both sides of the paper.
Heretofore no sheet-fed press had the capability of printing on two sides of a sheet in as many as four colors by passing the paper through the press one time.
It is the common and accepted practice in the printing industry to run a sheet to be printed through the sheet-fed press a multiplicity of times to attain multicolor printing on two sides of a sheet. After each pass of the sheet through the press, the plates must be changed and the press made ready for the next pass to apply other colors or to print on the back of the sheet. It is readily apparent to those skilled in the printing art that a considerable amount of time is spent making sheet-fed presses ready to print and in attaining proper registry of the numerous components of the press.
In a typical four-color one-side printing press a sheet delivered from the feeder is caught by the gripper bars of a first transfer cylinder. The sheet is folded around the transfer cylinder and carried to the grippers on the first impression cylinder where the grippers of the transfer cylinder release the paper and it is caught by the grippers of the impression cylinder. The grippers on the impression cylinder rotate the paper into contact with the blanket cylinder where printing is accomplished in one color on one side of the sheet. When the grippers on the impression cylinder release the sheet, grippers on a second transfer cylinder grasp the sheet, causing the printed surface to be in contact with the transfer cylinder while it is rotated to the grippers of a second impression cylinder. The grippers of the second transfer cylinder release the sheet as it is caught by the grippers of the second impression cylinder which rotates the sheet into contact with a second blanket where a second color is applied to the same side of the sheet. Grippers on a third transfer roller catch the sheet as it is released by the grippers of the second impression cylinder and the printed surface is again brought into contact with a transfer cylinder while it is being delivered to the grippers of a third impression cylinder. This process is continued until the sheet passes to delivery. When one side of the sheet is completed, the press is replated, the sheets are turned and re-fed through the press to print the other side of the sheet.
Virtually all sheet-fed printing presses heretofore developed have the characteristic of feeding the sheet serpentine fashion through the press while the grippers associated with each cylinder catch the sheet as it is being released by the grippers of the previous cylinder.
One of the major problems encountered by the printing industry lies in synchronizing the various cylinders whereby the sheet will be grasped and released at the proper moment for maintaining registry between the cylinders of successive towers so that colors do not overlap or separate.
Chains have been used in the past with limited success to transfer sheets from one printing station to another. Grippers supported by the chain have to be positively indexed to the printing station cylinders before sheet transfer can be accomplished with any degree of register between stations.
A chain has inherent limitations as a smooth transfer media because chordal motion of the links limit smooth flow; linear deformation of the chain results from numerous pivot joints; lubrication requirements at joints, to help prevent wear, noise, shock and vibration, present maintenance problems.
The gripper and chain transfer media could not, by itself, register the sheet between printing stations, even with the chain travelling precisely at cylinder speeds. As a compromise, grippers had to be loosely supported on the chain, moved from normal position, and indexed to printing station cylinders prior to actual sheet transfer at the cylinder. As soon as sheet transfer was accomplished and the gripper became separated from index with the cylinder, the gripper jumped or jerked back into its normal relation with the chain.
In the transfer system employed and disclosed herein, there is no contact between tape directed gripper bars and the printing cylinders thereby eliminating shock, vibration, wear, noise, mis-register and the other apparent problems accompanied by chain supported grippers being indexed to cylinders. The printing cylinders are entirely independent of the sheet transfer mechanism and vice versa except for speed synchronization of cylinder surface speed with that of the tape.
Another problem has been the offsetting of wet ink on transfer cylinders from the freshly printed surface on the paper and consequently back on to the next sheet that is passed through the press. Heretofore, presses with a multiplicity of towers for applying more than one color of ink to the sheet were driven by a common drive through a complex gear train or through long shafts which have inherent distortion thereby increasing the problem of synchronizing components of the press thereby making precision registry more difficult.
Typical four-color one-side printing presses have an average of about twenty cylinders including the plate cylinders, blanket cylinders, impression cylinders, transfer cylinders and skeleton wheels.
Sheet-fed printing presses heretofore used have relatively low production speeds which never exceed eight thousand impressions per hour.
All sheet-fed presses heretofore used have basically the same complex ink fountain with keys to vary the ink flow and an ink train consisting on an average of about twenty rollers for smoothing and distributing the ink to the plate cylinder.
A universal characteristic of sheet-fed printing presses heretofore used has been the employment of massive bearers on each end of the plate and blanket cylinders to assure rotation of the cylinders without vibration when the cylinder load is reduced because of gaps in the cylinders. The use of bearers has been necessitated by limitations of bearings heretofore incorporated into the design of presses for journaling the cylinders.
Apart from the equipment design being basically the same, one only has to be briefly associated with problems in the industry to see that printing problems, too, are the same for the similarly designed presses; namely, extensive time and effort are required for make-ready; extreme difficulty in obtaining and maintaining register between colors; streaking and slur caused by gear lash and deformation or by vibration and shock of complex mechanism movements; offsetting caused by the printed side of the sheet being in contact with transfer cylinder and skeleton wheel surfaces; sheet or board fatigue; considerable downtime for maintenance caused by breakdown of the complex mechanical systems; problems relating to ghosting on certain printing layouts; problems relating to control of ink-water balance and sometimes the most neglected problem of all, that of requiring personnel having special skills, talents, experience and perserverance to "get the job done" with the above mentioned type of printing systems.
All the above problems are related basically to problems involving lack of versatility, quality, economy and ease of operation, and are largely caused by the stereotype conventional design of the present day printing system.
Since the problems for the sheet-fed printer are not being readily solved by "updating and face-lifting" of the old concepts of printing, the only apparent alternative has been to switch to web-offset lithography. Here the printer can print several colors on two sides of the sheet at the same time with increased production. In addition to the multi-color perfecting capability the web-press is superior to the sheet-fed press in specific situations because higher production rates and lower break-even points are possible.
This at first would seem to be the answer, except for the fact that many of the problems existing in sheetfed printing also exist in web-offset; namely, lack of color register caused by deformation of long drive shafts; basically the same kind of ink fountain with keys used in sheet-fed presses; a complicated train of rollers and conventional water fountain systems; common drive for the entire press; roller or ball-bearings with massive cylinder bearers on the plate and blanket cylinders; and printing cylinders are universally the same circumference as the finished sheet cut-off length, allowing absolutely no time for recovery of the inking form rollers after they finish a printing cycle.
Apart from problems common to the conventional sheet-fed operation, switching from sheet-fed to web-offset lithography presents other distinct disadvantages.
A web-offset press is limited to one sheet length equal to the circumference of the plate cylinder. When shorter sheet lengths are required excessive waste results from non-use of the unprinted web portion. Another complete press system must be designed, manufactured, purchased and used for printing different sheet sizes to avoid excessive waste of paper. Web presses are generally more expensive because of complex folders, dryers, chill devices, etc., necessary. More time is usually required for make-ready and more waste is encountered since the web must be running through the press and desirably at production speeds while registering and while color correction changes are being made because it is difficult to compensate for wind-up of the drive system when the press is stopped. Crews trained for printing on sheet-fed equipment find that they must learn new skills when using web equipment.
The printing industry is faced with a dilemna of the sheet-fed and web-fed printing operations, each having decided advantages over the other, while sharing common problems which are inherent in the stereotyped press design which has been virtually unchanged for decades.